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The Mesmetron is an experimental pre-War "non-lethal pacification" weapon that was being tested in the months immediately preceding the Great War. It operates by sending a signal pulse into a human target's brain that scrambles higher-level mental functions, leaving the target extremely susceptible to suggestion. Still in its testing phase, the weapon was found to have the potential to trigger two undesirable side-effects: extremely increased aggression, or spontaneous cranial eruption.

When used on the specified quest non-player characters, it puts them into a hypnotic stupor, allowing the player character to rob and enslave them (by putting a slave collar on them) and send them to Paradise Falls. It also appears to work on many other non-player characters (aside from quest related ones), but can simply send them into a berserk frenzy in which they turn hostile to the Lone Wanderer and every other non-player character in the area. Occasionally, victims will run several steps before their heads explode.

It uses Mesmetron power cells as ammunition, starting out with 50 power cells. Grouse will also sell additional power cells, but at the fairly high price of 200 caps for 10 power cells. This gives the opportunity to make an additional 2500 caps every time one purchases more power cells (provided all 10 slaves were successfully captured). An additional 71 Mesmetron power cells can also be found in the underground lab in the Point Lookoutadd-on.

As previously mentioned, using the Mesmetron on a non-player character can have 3 different outcomes. These effects vary and are generally unreliable, so it is advised to save before using this weapon on anyone important. Due to the fact that using the Mesmetron is considered assault, it is advised to "mezz" someone while undetected (Stealth Boys and the Chinese stealth armor will come in handy here). Be aware that mezzing anyone who is already wearing a slave collar will cause the collar to malfunction and explode, killing the victim. Hostile non-player characters who were mezzed can also be turned friendly by being freed by the player character (high Science skill required).
An analysis of the Mesmetron's effects:

This effect stuns the target for a few seconds, during which time the victim will stand with their head swaying from side to side and it is possible to engage them in conversation. Some characters cannot be stunned, in which case the probability of the Frenzy effect is raised to 80%.

While in conversation, there are three dialogue options: The first convinces the character to allow having their things taken (by either telling them it's a magic trick, that there is a thief on the loose, that the player character thinks they stole something from them, or by just telling them to give their things) and will open up the victim's inventory (even what they are wearing/using), allowing the player to remove or add any item. The second slips a slave collar on the character (instantly netting negative Karma). The third simply tells the victim to "walk it off"; this will exit the conversation and make the character resume what they were doing (assuming no onlookers turned hostile).

The second set of dialogue options is unlocked by slipping a collar onto the victim: The first is the "cruel" way to tell the victim that they are now a slave ("...your head will explode!"), the second option allows access to the slave's inventory, and the third option is a slightly friendlier way to inform the victim of their "new occupation" ("...nothing personal").

If the character is not spoken to before the stun effect wears off, they will turn hostile towards anyone they can see, that includes the player character.

This second effect causes the victim to go into a state of frenzy, attacking anyone nearby (most likely the player character), and causing all other characters to turn on them. The onlookers may also turn hostile to the PC if caught stunning the victim. This method can also be a reliable way to get rid of victims without attacking them or getting negative Karma. This is dangerous, however, especially if one of the enemies has any high-damaging weapons.

This final effect of the Mesmetron may not be the most useful, but is definitely the most spectacular. When victims are affected by the Mesmetron in this way, they will turn hostile (and either run, stand still, or attack) for just a moment before their heads literally explode. This effect will instantly give negative Karma (if the victim was good/neutral) and turn any nearby non-player characters hostile to the player character.

If deciding not to take the quest, the Mesmetron can be obtained by killing Grouse and looting his corpse. It cannot be stolen from him, meaning he needs to be dead to get it. If this is done, it's no longer possible to purchase any Mesmetron power cells nor slave collars from him, thus one can't enslave non-player characters. The Mesmetron can still be exploited to frenzy or stun and rob non-player characters, however.

Robobrains have this weapon built-in, though it only causes damage, not frenzy or hypnosis.

Enslavable targets are based on a faction list of 672 members, filled mostly with generic non-player characters. There's a shorter faction list of non-player characters who can always be mezzed, and this initially includes the four objectives of the Strictly Business quest and one generic leveled slave. Essential non-player characters may be immune.

A (possibly incomplete) list of enslavable non-player characters is located on the Strictly Profitable page.

After a non-player character is successfully mezzed they are added to a faction list that can never be mezzed.

A slave collar is required to enslave someone. Only one collar can be carried and therefore only one person may be enslaved at a time.

It appears that unarmed (or disarmed) non-player characters are more likely to be enslaved by the Mesmetron. Attacking a non-player character's weapon or arms and then switching to the Mesmetron may be an effective technique.

Using the Mesmetron in V.A.T.S. can result in the Mysterious Stranger appearing if having the perk. The Mysterious Stranger will not fire on the target if they were successfully stunned, though V.A.T.S. will remain active for the usual period of time.

Non-player characters can be enslaved even if all of the slavers in Paradise Falls are dead.

Any enslaved raiders will almost never get to Paradise Falls alive even if fast traveling there.

The game will keep track of how many characters were "mezzed". This information can be accessed on the Pip-Boy.

Generic non-player characters (such as raiders) who are enslaved and successfully sent to Paradise Falls will not respawn.

One must consider if they want to interact with the character again before enslaving them.

Significant use of the Mesmetron in a specific area can permanently clear that area of human enemies.

Additionally, enslaving a generic, hostile non-player character and then subsequently removing the slave collar will cause the non-player character to turn friendly to the Lone Wanderer and companions (other alliances/hostilities, e.g. Brotherhood of Steel vs. Enclave, are not affected). This change persists even if the non-player character is killed and later respawns. Affected non-player characters will attack their usual allies (e.g. other Enclave members) if those allies attack the player character.

When used in conjunction with the Animal Friend and Robotics Expert perks, repeated use of this weapon to enslave NPCs and then freeing them can be exploited to gain control over entire factions, even those that are hostile or otherwise unjoinable, such as raiders, the Brotherhood Outcasts, and even the Enclave. However, doing so will net a high amount of negative Karma, and some named/non-humanoid members (e.g. Enclave-controlled deathclaws, sentry guns. etc.) of these factions will still be potentially hostile towards the player.

From time to time a mesmerized and collared target won't make it to Paradise Falls. This causes the collar to be lost and as no slave was turned in, the collar must be re-purchased from the slavers. To ensure success, fast-travel to Paradise Falls if possible. (see bugs for more)

Followers may briefly attempt to attack a non-hostile non-player character once that NPC is stunned; this is because the non-player character will temporarily turn hostile to the Lone Wanderer before the stun effect kicks in.

"Mezzing" anyone in the Citadel Laboratory will automatically frenzy them (after the quest "The American Dream"). Witnesses will not turn hostile to the player.

If the Mesmetron is used on an unfreed captive and they are then freed, it is then impossible to interact with them again, except to mezz or kill them.

It is to be noticed that in third person the Mesmetron has no trigger.

The Mesmetron's light is visible when the stealth field of a Stealth Boy or the Chinese stealth armor is in use. It will not be on, however, if it is equipped (not unholstered) after the stealth field kicks in. (The light is always on in 1st person. In 3rd person, it can vary depending on how the Lone Wanderer equips it.)

There is a mention of the Mesmetron in Fallout 4 on a terminal at the second floor of the HalluciGen, Inc. building, indicating that the lead engineer for its development also worked for HalluciGen.

PC Freshly enslaved non-player characters will often be reported by Grouse to have died once the Lone Wanderer arrives at Paradise Falls. This is caused by a condition in the Mesmetron effect script which automatically moves the slave to Paradise Falls—indirectly marking them as dead in the process—once the non-player character is no longer being actively rendered. Aside from leaving the non-player character's immediate vicinity, numerous actions have a random chance to cause an non-player character to stop being rendered, including accessing the Pip-Boy, facing away from the non-player character for a certain period, or even accessing the game menu. [verified]